The monument has been straightened by half a degree, stabilising it for the first time in more than eight centuries.

"Apart from seasonal, cyclic movements, the tower has been basically motionless since September 2003. We believe geotechnical stabilisation has been achieved," Jamiolkowski said.

Cyclic displacements include the tower heating up at sunrise and leaning slightly to the west before returning to the original position.

The soft, sandy subsoil is what has given the tower, which is more than 50 metres high, its lean since Bonanno Pisano began building it in 1173.

The present scheme involved several experimental techniques, including putting several hundred tonnes of lead counterweights on the side opposite the lean.

At the end, experts used the simplest and most intuitive solution: digging out some earth from the side away from the tilt to allow the tower to settle more evenly.

More than 40 drills were used to remove 38 cubic metres of earth. Meanwhile the tower, built using thousands of tonnes of intricately carved white marble, was steadied by steel cables attached to the first tier and anchored to the ground.

She'll be right, said masons

The tower first started to subside when it was about nine metres high. Undeterred, masons continued the work, adding more levels and using columns of varying length in the vain effort to straighten the slant as the tower grew higher.

Pisa's most famous landmark was completed 180 years later, with the bell tower placed on top in 1350.

Since then, no fewer than 17 committees have debated on how best to correct the monument's increasingly drunken angle. The archives of the Opera Primaziale, the body responsible for the tower's care, are full of schemes proposed over the past 150 years.

Professor Carlo Viggiani of the University of Naples said the key point of the current project was finding out that a slight decrease in inclination was all that was needed to stabilise the tower.

The restoration has made the tower safe for the next 300 years, an achievement unimaginable 12 years ago, when the monument was so far off perpendicular that it risked collapsing.

The tower was closed from January 1990, after experts noticed that its inclination had increased at a rate of 1 millimetre a year. The straightened tower reopened to the public in December 2001.

Guided tours of 30 people at a time are allowed to climb the 294 steps up the spiral stairway to the bell chamber.